Wireless networks - Lecture 22: WCDMA (Part 2)

Wireless networks - Lecture 22: WCDMA (Part 2). The main topics covered in this chapter include: spreading and scrambling; transport channels; physical channels; signalling; physical layer procedures; orthogonal variable spreading factor (OVSF) technique; coded composite transport channel (CCTrCh);...

Wireless Networks

Lecture 22 WCDMA (Part II)

Dr Ghalib A Shah

Outlines

 Last Lecture Review

 Spreading and Scrambling

Last Lecture Review

Spreading and Scrambling

 Spreading means increasing the signal bandwidth

 Strictly speaking, spreading includes two operations:

► Channelization (increases signal bandwidth) - using orthogonal

codes

► Scrambling (does not affect the signal bandwidth) - using

pseudo noise

Channelization

 Channelisation codes are orthogonal codes, based on

Orthogonal Variable Spreading Factor (OVSF)

technique

 The codes are fully orthogonal, i.e., they do not

interfere with each other, only if the codes are time

synchronized

 Thus, channelization codes can separate the

transmissions from a single source

 In the downlink, it can separate different users within

one cell/sector

 Limited orthogonal codes must be reused in every cell

► Problem: Interference if two cells use the same code

► Solution: Scrambling codes to reduce inter-base-station

interference

 It is possible that two mobiles are using the

same codes.

uplink, scrambling codes are used.

code on top of the tree.

Channelization

 In the scrambling process the code sequence is

multiplied with a pseudorandom scrambling code

 The scrambling code can be a long code (a Gold code

with 10 ms period) or a short code (S(2) code)

 In the downlink scrambling codes are used to reduce

the inter-basestation interference Typically, each Node

B has only one scrambling code for UEs to separate base stations Since a code tree under one scrambling code is used by all users in its cell, proper code

management is needed

Channel Concept

 Three separate channels concepts in the UTRA:

logical, transport, and physical channels

► Logical channels define what type of data is transferred

► Transport channels define how and with which type of

characteristics the data is transferred by the physical layer.

► Physical data define the exact physical characteristics of the

radio channel.

Trans port Channels  ­> Phys ical Channels

 Transport channels contain the data generated at the

higher layers, which is carried over the air and are

mapped in the physical layer to different physical

channels

 The data is sent by transport block from MAC layer to

physical layer and generated by MAC layer every 10 ms

 The transport format of each transport channel is

identified by the Transport Format Indicator (TFI),

which is used in the interlayer communication between the MAC layer and physical layer

 Several transport channels can be multiplexed together

by physical layer to form a single Coded Composite

 The physical layer combines several TFI information

into the Transport Format Combination Indicator

(TFCI), which indicate which transport channels are active for the current frame

 Two types of transport channels: dedicated channels

► Dedicated channel – reserved for a single user only.

• Support fast power control and soft handover.

► Common channel – can be used by any user at any time

• Don’t support soft handover but some support fast power control.

 In addition to the physical channels mapped from the

transport channels, there exist physical channels for

signaling purposes to carry only information between

network and the terminals

Transport Channel Physical Channel (UL / DL) Dedicated channel DCH Dedicated physical data channel DPDCH

Dedicated physical control channel DPCCH (UL) Random access channel RACH Physical random access channel PRACH

UL Dedicated Channel DCH

equipment caused from the discontinuous UL transmission, two dedicated physical channels are

► Dedicated Physical Control Channel (DPCCH)

► Dedicated Physical Data Channel (DPDCH)

to overcome discontinuous transmission

(DTX)

 Dedicated Physical Control Channel (DPCCH) has a fixed

spreading factor of 256 and carries physical layer control

information.

 DPCCH has four fields: Pilot, TFCI, FBI, TPC.

► Pilot – channel estimation + SIR estimate for PC

 Dedicated Physical Data Channel (DPDCH) has a spreading factor from

4 to 256 and its data rate may vary on a frame-by-frame basis informed

UL receiver in BS

 It performs following

► Start receiving the frame, de-spreading DPCCH and buffering

the DPDCH according to max bit rate corresponding to the smallest spreading factor

► For every slot

• Obtain channel estimate using pilot bits and estimate SIR

• Send TPC command in DL to UE to control UL tx power

• Decode TPC bit in every slot and adjust DL power for that UE

► For every 2 nd or 4 th slot

• Decode FBI bits, if present in 2 or 4 slots and adjust antenna phases and amplitude for transmission diversity

► For every 10 ms frame

• Decode TFCI information from DPCCH frame to obtain bit rate

UL Multiplexing and Channel Coding 

Chain

RACH Operation

 First, UE sends a preamble.

 The SF of the preamble is 256 and contain a signature sequence

of 16 symbols – a total length of 4096 chips.

 Wait for the acknowledged with the Acquisition (AICH) from the

BS.

 In case no AICH received after a period of time, the UE sends

another preamble with higher power.

 When AICH is received, UE sends 10 or 20 ms message part.

 The SF for the message is from 32 to 256.

Synchronis ation Channel (SCH) – Cell

Searching

 Cell search using SCH has three basic steps:

► The UE searches the 256-chip primary synchronisation code,

which is common to all cells and is the same in every slot

Detect peaks in the output of the filter corresponds to the slot boundary (slot synchronisation).

► The UE seeks the largest peak secondary synchronisation

code (SSC) There are 64 unique SSC sequences Each SSC sequence has 15 SSCs The UE needs to know 15 successive SSCs from the S-SCH, then it can determine the code group in order to know the frame boundary (frame synchronisation).

► Each code group has 8 primary scrambling The correct one is

found by each possible scrambling code in turn over the CPICH of that cell.

SSC Sequence

Secondary Synchronization Code (SSC) and Code Group Code group #0 #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14

.

.

16 6 9 16 13 12 2 6 2 13 3 3 12 9 7 16 6 9 16 13 12

Power Control

► Feedback information.

► Uplink PC is used for near-far problem Downlink PC is to

ensure that there is enough power for mobiles at the cell edge.

► One PC command per slot – 1500 Hz

► Two special cases for fast closed loop PC:

• Soft handover: how to react to multiple power control commands from several sources At the mobile, a “power down” command has higher priority over “power up” command.

• Compressed mode: Large step size is used after a compressed frame to allow the power level to converge more quickly to the correct value after the break.

 Closed Loop PC - Outer Loop PC

► Set the SIR target in order to maintain a certain

frame error rate (FER) Operated at radio network controller (RNC)

► No feedback information